A local measure of the wind speed can be useful for a number of applications, and in particular for monitoring the thermal behavior of power lines, since cooling by the wind component perpendicular to the power lines is one of the main factors in this thermal behavior.
As explained in U.S. Pat. No. 8,184,015, continuous monitoring of electrical power lines, in particular high-voltage overhead lines, is essential to timely detect anomalous conditions which could lead to a power outage. Measurement of the sag of power line spans between successive supports to determine whether the sag is lower than a maximum value is becoming a mandatory requirement in some countries.
U.S. Pat. No. 8,184,015 disclosed a device and method for continuously monitoring the sag on a power line span. This method allowed the determination of mechanical dynamic properties of the power lines just by sensing mechanical vibrations in a frequency range from 0 to some tens of Hertz. Indeed, power lines in the field are always subject to movements and vibrations, which may be very small but detectable by their accelerations in both time and frequency domains.
Such properties may also be used to determine many other features. The new method of the present invention can also be used by other devices equipped with accelerometers.
The maximum allowable constant electrical current rating which will meet the design, security and safety criteria, such as electrical clearance, of a particular power line on which an electrically conductive cable is used is known as the “ampacity”, as described for instance in “Sag-tension calculation methods for overhead lines”, published in 2007 as CIGRE Technical Brochure No. 324 by Study Committee B2 of The International Council on Large Electric Systems (CIGRE). Methods to evaluate the ampacity of a suspended cable span on the basis of various data are explained for instance in A. Deb's “Power line ampacity system”, published in 2000 by CRC Press, and in technical brochures from international organizations, such as CIGRE Technical Brochures No. 207 (“Thermal behaviour of overhead conductors”) and No. 498 (“Guide for application of direct real-time monitoring systems”), respectively published in 2002 and 2012, as well as in abovementioned CIGRE Technical Brochure No. 324. The methods disclosed in these documents use weather data as locally measured or simulated following international recommendations as explained, for example, in CIGRE Technical Brochure No. 299 (“Guide for selection of weather parameters for bare overhead conductor ratings”), published in 2006 or IEEE Standard 738-2006 for calculating the current-temperature of bare overhead conductors, published in 2006.
A drawback of these methods for measuring or simulating weather conditions is that none of them is able to generate appropriate weather data which are actually to be used to calculate ampacity, which is a value linked to all critical spans of a power line. A critical span is a span for which there is a significant risk of potential clearance violation in any kind of weather situations. Which spans are critical may depend i.a. on the span orientation, local screening effects, and local obstacles such as vegetation, buildings, roads, etc. They are normally defined at the design stage but may be reviewed by more modern techniques like Light Detection And Ranging (LIDAR) survey.
The wind speed has a dramatic impact on power line ampacity as it is the main variable responsible for cooling down the conductor, and hence for the sag value. However, wind speed measurement is complicated for various reasons. Firstly, wind speed is not stationary and can vary significantly within minutes, apart from the sudden changes linked to wind gusts. Secondly, wind speed also varies along the span. For example, according to Simiu E. and Scanlan R., in “Wind effects on structures”, published by John Wiley & Sons, Inc in 1996, wind vortices have a typical average size of several tens of meters. Therefore, a typical suspended cable span of a power line, with a length of several hundreds of meters, is subject to a variable wind speed along this length. Thirdly, the wind speed also greatly varies in a vertical direction, since the conductive cable span is suspended within the atmospheric boundary layer, and the lowest point of the suspended cable span is generally about 10 meters over the ground. The wind speed may also vary due to local effects, such as screening from trees or buildings or the height of the suspended cable, which may change in a single span by more than 15 meters just by its sag, but may additionally be subject to ground level differences between the end points of the span. Because the cable is suspended in the atmospheric boundary layer, such local differences in height near the ground can have significant effect in the wind speed. Therefore, a single-spot wind speed measurement is normally insufficient to compute the global effect of the wind over the whole span.
All of these factors are particularly important for low wind speeds, in particular for wind speeds whose component perpendicularly to the conductor axis is lower than 3 m/s. Such low wind speeds are critical for ampacity determination. Similarly, a single-spot measurement of the effective incident radiation does not allow computing the global effect of the combined effect of sun and albedo over the whole span.
Given the importance of power line monitoring, several devices have been proposed to measure at least some of the relevant parameters. For example, in the second edition of the EPRI Transmission Line Reference book “Wind induced conductor motion”, published in 2009, it was disclosed to use displacement measurement systems placed at a given short distance (e.g. 89 mm) from a cable suspension point in order to measure high-frequency vibrations. However, this is only a partial solution to the monitoring problem and such systems are solely oriented to evaluate the life time of power line conductors due to the bending fatigue induced by Aeolian vibrations cycles on conductor strands near clamps.
A number of different methods to measure the sag of a suspended cable span are also known. An example of tentative sag measurement consists in the optical detection of a target clamped on the monitored conductor by a camera fixed to a pylon, as disclosed in U.S. Pat. No. 6,205,867. Other examples of such methods include measurement of the temperature, tension or inclination of the conductor in the span. A conductor replica is sometimes attached to the tower to catch an assimilated conductor temperature without Joule effect. Besides the fact that these methods only allow a partial monitoring of the power line, such methods suffer from other drawbacks: optical techniques are sensitive to reductions of the visibility induced by meteorological conditions while the other measurement methods depend on uncertain models and/or data which may be unavailable and/or uncertain, e.g. wind speeds, topological data, actual conductor characteristics, etc.
U.S. Pat. Nos. 5,140,257 and 5,341,088 disclose a monitoring device whose housing is attached to the conductor. Some features of this device are related to the measurement of wind speed and direction based on hot wire anemometers. However, hot wire anemometers are extremely difficult to manage in such close proximity to a high-voltage power line. Moreover, the measured wind speed may be altered by the sensor itself, since the hot wire needs to be protected against corona discharges.
U.S. Pat. Nos. 6,441,603 and 5,559,430 disclose a monitoring device for rating an overhead power line, which is not attached to the conductive cable of the power line, but replicates it instead. The evaluation of the combined effect of wind, solar radiation, albedo, and ambient temperature is based on the behavior of dedicated rods installed separately from the conductive cable. The drawback of such a local replication, however, is that the variations in wind speed and effective incident radiation along the span are not taken into account.
Consequently, such a local measurement may not be a good indication of the actual mean wind speed and global incident radiation along spans of several hundreds of meters with possibly variable heights and variable winds along the span. Moreover, using a replica may cause additional errors with respect to the mean values of conductor emissivity and absorptivity and global incident radiation along the whole span of suspended conductive cable.
U.S. Pat. No. 4,728,887 discloses a monitoring device whose housing is adjacent to an overhead line. However, there is no information about how wind speed and its direction may be taken into account to evaluate ampacity.
U.S. Pat. No. 5,933,355 discloses software to evaluate ampacity of a power line. This software, however, has no relationship with wind speed measurement.
U.S. Pat. No. 6,205,867 discloses a power line sag monitor based on inclination measurement. There is however no information about how wind speed and direction may be measured to calculate ampacity.
PCT Patent Application Publication WO 2010/054072 is related to real time power line rating. It alleged the existence of a sensor about wind speed direction and amplitude but did not disclose how these sensors are constituted.
PCT Patent Application Publication WO 2004/038891 and Norwegian Patent Application Publication NO 20024833 disclose a monitoring device whose housing is attached to a suspended cable. The wind is measured by “a traditional wind gauge” and that such wind gauge “operates with an opening in the outer casing”. However, a drawback of traditional wind gauges is that the gauge itself constitutes a perturbation in the local measurement and low wind speeds cannot be measured properly by such a gauge.
European Patent Application Publication EP 1.574.822 discloses a monitoring device whose housing is attached to a suspended cable. There is however no information about how wind speed and direction may be taken into account to evaluate ampacity.
Korean Patent Application Publication KR 2009/0050671 discloses a monitoring device whose housing is attached to the conductor. However, this document does not disclose how the effective wind speed, perpendicularly to the conductor, can be measured below 3 m/s, which is the basic case for ampacity determination.
U.S. Patent Application Publication US 2012/0029871 A1 discloses a monitoring device whose housing is attached to the conductor. However, this document does not disclose how to evaluate the wind speed to consider for ampacity determination. On the website http://www.lindsey-usa.com/newProduct.php, it is stated that the sensors in one such monitoring device may be tasked to detect “galloping” and Aeolian vibration, which is an indication of wind blowing across the suspended cable.
In the abovementioned books “Wind effects on structures” and “Wind induced conductor motion” it was already disclosed that Aeolian vibration frequencies are linked to wind speed. Furthermore, in the articles “Original Real-time Observations of Aeolian Vibrations on Power-Line Conductors” and “Aeolian Vibrations on Power-Line Conductors, Evaluation of Actual Self Damping”, both published by Godard, B, Guerard, S, & Lilien, J.-L. in IEEE Transactions on Power Delivery, 26(4), 2012, it was also disclosed that, since Aeolian vibrations are generated by von Karman vortex shedding, perpendicular wind speed can be calculated following the Strouhal formula:
  Str  =            f      ·      d        u  wherein Str is a dimensionless Strouhal number characteristic of the cable, f is an Aeolian vibration frequency, d is a cross-sectional diameter of the cable, and u a perpendicular wind speed component relative to the suspended cable span.
Although it is thus known that the perpendicular wind speed component can be measured through Aeolian vibration frequencies, the drawback remains that such Aeolian vibrations are normally only present at certain wind speed ranges, and in particular at relatively low wind speeds. Even though these ranges are most critical, for instance, for the determination of the ampacity of an overhead power line, the drawback remains that is not possible to measure the perpendicular wind speed component across all wind speed ranges based on Aeolian vibration frequencies only.